Eight million US adults have kidney disease (Fox et al., (2004) JAMA, 291: 844-850). To develop strategies for prevention and treatment of renal diseases, it is essential to study GFR (glomerular filtration rate), ERPF (effective renal plasma flow) and the filtration fraction (GFR/ERPF). These measurements represent important variables in the investigation and understanding of renal pathophysiology, as well as common diseases such as nephrotic syndrome, hypertension, and diabetes. Such studies enhance our understanding of the pathophysiology of chronic renal injury in humans, help elucidate the mechanisms by which chronic injury progresses to end-stage renal disease, and lead to improved strategies for prevention and therapy (Gellermann & Querfeld (2004) Pediatr. Nephrol. 19: 101-104; Fesler et al., (2004) Hypertension 43: 219-223; Torbjornsdotter et al., (2004) Diabetes Care 27: 510-516).
There is a continuing need for novel renal tracers with a capacity to measure effective renal plasma flow comparable to the current gold standard of para-aminohippurate (PAH). Physicians typically assume that 131I-ortho-iodohippurate is an excellent agent for the non-invasive measurement of ERPF since it is much easier to measure in plasma and urine than PAH, and there is no risk that the tracer quantities of 131I-OIH will saturate the organic anion transporter and lead to spuriously low measurements of ERPF. A serious problem with 131I-OIH, however, is that 15-20% of the 131I-OIH administered to a patient binds to red blood cells, and the 131I-OIH must leave the red cells before it can become available for renal tubular extraction. Consequently, when one collects a renal vein blood sample to determine the 131I-OIH extraction fraction, 131I-OIH leaves the red cells and re-enters the plasma during the period required to separate the plasma from the red cells, thus leading to a spuriously high measurement of the plasma 131I-OIH concentration (Rehling et al., (1995) Eur. J. Nucl. Med. 22: 1379-1384). Agents with near zero binding to red blood cells are, therefore, preferred. Additionally, 131I-OIH is also no longer commercially available in the United States.
For imaging purposes, the first-generation renal tracer technetium-99m mercaptoacetyltriglycine (99mTc-MAG3) has now replaced 131I OIH. For measuring ERPF, however, 99mTc-MAG3 is still far from optimal. The clearance of 99mTc-MAG3 is only 50-60% that of 131I-OIH (see, for example, Taylor et al., (1989) Radiology 170: 721-725; Schaap et al., (1988) Eur. J. Nucl. Med. 14: 28-31; Jafri et al., (1988) J. Nucl. Med. 29: 147-158; Russell et al., (1988) J. Nucl. Med. 29: 1931-1933; Bubeck et al., (1990) J. Nucl. Med. 31: 1285-1293; Blaufox et al., (1996) J. Nucl. Med. 37: 1883-1890; O'Reilly et al., (1996) J. Nucl. Med. 37: 1872-1876). Volkman et al., (In Radionuclides in Nephrology; Edited by O'Reilly et al., 1994; pp. 21-26) concluded that 99mTc-MAG3 is not suitable for the accurate estimation of the renal plasma flow because there are marked variations of the extraction fraction of 99mTc-MAG3 in kidneys although displaying a constant extraction fraction of PAH. For example, 99mTc-MAG3 clearances in 12 volunteers one week apart showed that the 99mTc-MAG3 clearance had to change by 35% before a clinician would be 95% confident that the measured change represented a real change in renal function.
Several factors may account for the marked variability of 99mTc-MAG3 clearances compared to PAH. Fasting rats have almost twice the hepatobiliary clearance of 99mTc-MAG3 as rats with free access to food: 11.7% versus 6.6%, respectively. However, variations in hepatic transport with meals are not sufficient to explain the observed variability in 99mTc-MAG3 clearance since it has also been shown that only 0.5-1.0% of the injected 99mTc-MAG3 is transported into the small intestine via the hepatobiliary system in normal volunteers. This percentage increases in patients with renal failure. Similarly, variations in the radiochemical purity following kit reconstitution are an unlikely explanation since it has been shown that the 95-96% radiochemical purity of the European 99mTc-MAG3 kit is comparable to the US kit (Eshima et al., (1997) J. Nucl. Med. 38: 49P; Murray et al., (2000) J. Nucl. Med. Commun. 21: 71-75).
The most likely factor contributing to the clearance variations is a variation in protein binding. Protein binding of 99mTc-MAG3 in rats is sensitive to physiological changes. For example, the protein binding of 99mTc-MAG3 decreased from 78% to 44% following PAH infusion and decreased to 42%, 63% and 65%, respectively, following mannitol, ammonium chloride and sodium bicarbonate infusion. A decrease in protein binding secondary to drugs, accumulation of organic acids, or a change in the physiological state could increase the 99mTc-MAG3 clearance by making a higher fraction of the 99mTc-MAG3 available for both glomerular filtration and tubular transport.
Another ERPF99mTc tracer is 99mTc-LL-ethylene dicysteine (Vanbilloen et al., (2000) Nucl. Med. Biol. 27: 207-214; Vanbilloen et al., (1996) Eur. J. Nucl. Med. 23: 40-48; Cleynhens et al., (1991) J. Nucl. Med. 32: 1016; Bormans et al., (1988) J. Nucl. Med. 29: 909; Miller et al., (1989) J. Nucl. Med. 30: 937-938). However, although 99mTc-LL-ethylene dicysteine and 99mTc-DD-ethylene dicysteine have a higher clearance than 99mTc-MAG3 in humans, the clearance of 99mTc-DD-ethylene dicysteine, was still only about 60-65% of that of PAH (Taylor et al., (1997) J. Nucl. Med. 38: 821-826). These agents do not provide a reliable means of monitoring renal function because the solution structure of both DD- and LL-ethylene dicysteine exist in carboxyl ligated monoanionic (20%) and deligated dianionic (80%) forms in rapid equilibrium at physiological pH. They differ markedly in structure and charge, have different rates of clearance and different protein binding affinities, and consequently are affected differently by changes in pH, drugs that affect protein binding, and different renal pathologies. For sequential monitoring of renal function, therefore, a radiopharmaceutical to measure ERPF should exist as a single species at physiological pH.
Finally, a tracer may have a high affinity for an organic anion tubular transporter; however, if the tracer's plasma protein binding is high, it may be relatively unavailable to the transporter, leading to a low EF and, consequently, a low clearance (Eshima et al., (2000) J. Nucl. Med. 41: 2077-2082). Bubeck has postulated that the peritubular transit time is too short for complete dissociation of the highly protein-bound tracers such as 99mTc-MAG3 and 131I-OIH (Bubeck et al., (1990) J. Nucl. Med. 31: 1285-1293).
There is, therefore, a continuing need for renal tracers with an improved capacity to measure effective renal plasma flow (ERPF) comparable to the gold standard, para-aminohippurate (PAH).